Method of producing a corrosion-resisting coating on metallic molybdenum surfaces

ABSTRACT

Method of producing surface layers of bromide on molybdenum elements such as those used in incandescent halogen lamps. The surface of the molybdenum elements is degreased, the elements are heated in a stream of hydrogen at approximately 700*C for at least 10 minutes, and the elements are then treated with brominecontaining gas at about 1000*C for 10 minutes. Subsequently, the treated elements are cooled in a gas such as nitrogen.

United States Patent l-langos et al.

[ Sept. 24, 1974 METHOD OF PRODUCING A CORROSION-RESISTING COATING ON METALLIC MOLYBDENUM SURFACES Inventors: Istvan Hangos; Laszlo Varkonyi;

Jozsef Hodacs; Neuhof-Suski, all of, Budapest, Hungary Assignee: Egyesult Izzolampa es Villamosagi Reszvenytarsasag, Budapest, Hungary Filed: July 31, 1972 Appl. No.: 276,754

Foreign Application Priority Data Sept. 14, 1971 Hungary EE 1950 US. Cl 148/63, 313/318, 313/222,

204/56 R Int. Cl. HOlk 3/00 Field of Search 313/222, 315, 316, 318;

[56] References Cited UNITED STATES PATENTS 3,470,410 9/1969 Patsch.... 313/315 X 3,549,933 12/1970 Smalley. 313/315 X 3,668,391 6/1972 Kimball 313/315 X 3,719,853 3/1973 Sugaro et a1 313/222 3,723,792 3/1973 Chiola et al. 313/315 Primary Examiner-Ralph S. Kendall [5 7] ABSTRACT 6 Claims, N0 Drawings METHOD OF PRODUCING A CORROSlON-RESISTING COATING ON METALLIC MOLYBDENUM SURFACES The invention relates to a method of applying coatings of molybdenum bromide to the molybdenum components of halogenfilled incandescent lamps. In this method, the molybdenum surface is first deoxidized and then annealed in H Thereafter, the surface is treated with bromine vapour, e.g. in a stream of such vapour, for the purpose of forming a layer of bromine. The method of the invention is advantageously used in the production of halogen-filled incandescent lamps.

It is well-known that metallic molybdenum is used on a large scale in the manufacture of vacuum-containing products, in particular lamps and electron tubes. This is because of its very advantageous properties are regards vacuum technology, e.g. high melting point, low tension, adequate ductility, non-deformability at high temperatures etc. However, the metal itself is not capable of resisting attack by some gases, in particular oxygen, air and halogens, and the properties of the compounds or products formed in this way have an adverse effect upon the quality of the vacuum products. This problem is of particular importance in the case of lamps, for example halogen-filled incandescent lamps, where the corrosion of molybdenum can cause trouble during operation of the lamp, and in the case of halogen-filled incandescent lamps, for example, such corrosion prevents the completion of the halogen-tungsten cycle, and in more serious cases, the initiation of this cycle lHungarian Patent Specification No. 154 663/.

The invention is concerned with the production of a corrosion-resisting protective coating on metallic molybdenum surfaces used in vacuum-containing products, which coating forms, on the surface, a uniform cohesive covering which cannot be penetrated by corrosive gases.

Numerous methods are known for the protection of molybdenum surfaces against corrosion; for example a coating of noble metal is formed on the surface by electrodeposition, and this prevents corrosion at low temperatures.

The same object is achieved with a chromium coating. Such methods have been disclosed in the technical literature. If however the molybdenum is subjected to the effects of higher temperatures during operation of the vacuum-filled product, a considerable part of the coating diffuses from the surface into the interior of the material, and the corrosion-resisting effect is lost.

In other known methods /see e.g. Austrian Patent Specification No. 253 620/, molybdenum carbide is formed on the molybdenum surface by the deposition of carbon, and the carbide provides a corrosionresisting coating. This method however is expensive and does not ensure that resistance to corrosion continues when the surface is damaged.

The problem underlying the invention is that of providing a corrosion-resisting protective coating which can be applied by a simple method, is not expensive, and is automatically regenerated in the event of its being damaged. Thus, effective protection against corrosion is achieved by the protective layer applied to the molybdenum surface, irrespective of whether the surface is damaged during assembly of the product.

According to the invention, the surfaces of materials having a molybdenum structure and used in vacuumcontaining products are provided with a protective layer of molybdenum bromide. This protective layer is easy to produce and it has a low vapour pressure; a strong, heat-resisting, gas-proof coating is formed on the molybdenum surface, which coating has a selfregenerating action at any points where damage occurs, in the halogen-containing enclosure, i.e. the coating is able automatically to reproduce itself at the damaged points.

The metallic molybdenum forms the following prodv ucts with bromine:

Mo Br MoBr /at about 200C/ /l/ Mo Br, MoBr, /200 300c/ /2/ Mo Br, MoBr /300 500C] /3/ Mo Br (MoBr )x [above 600C! /4/ wherein x =1 .5.

For the present method, reaction /4/ is preferred, since the components of the reactions 1 3 can be converted into molybdenum bromide during operation of the lamp or even during the process of forming the protective layer. MoBr is heat-resisting, its vapor pressure is low, and in the event of damage it exercises, together with the bromine resulting from the abovementioned reactions, a self-regenerating action, i.e. it forms molybdenum bromide at the points where damage occurs.

Since the oxybromides of molybdenum are volatile and the formation of the complete protective layer is thus inhibited, it is expedient to deoxidize the surface carefully before forming the corrosion-resisting protective layer.

This can be done with the aid of one of the known methods, e.g. by reduction in pure hydrogen and heating, by cathodic reduction, pickling, etc.

The corrosion-resisting protective layer can likewise be applied in different ways, using processes known in chemical technology.

One of the molybdenum bromides obtained by the reactions 1 4 can be applied by, for example, deposition, painting dusting, or by electrolytic methods. The protective layer can also be formed on the surface by, for example, anodic bromination in a solution containing bromide ions, or by direct bromination on the surface at a suitable temperature, or by a heterogeneous gas-phase reaction, and so on. In each case the protective layer obtained is converted into a corrosionresisting layer of (MoBr by heating for a short period at 700C.

This can be done at any stage in manufacture, and preferably immediately prior to manufacture of the vacuum-containing product, or in the finished halogencontaining lamp itself,

The following examples illustrate ways of performing the method of the invention:

EXAMPLE 1 The machined and degreased molybdenum surface is first chemically cleaned in known manner, and the surface is then heated at 900C in pure dry hydrogen for up to 10 minutes immediately prior to the bromination process, for the purpose of deoxidizing it. The molybdenum components to be coated are then placed in molybdenum boats in a tubular furnace, and the furnace is flushed with nitrogen, for example, and raised to a temperature of approximately 600C. Thereafter, hydrogen is introduced into the furnace and the temperature therein is raised to about 700C for approximately minutes. Flushing with nitrogen is repeated, and bromine vapour is caused to flow through the furnace chamber at about lOOOC for a period up to 10 minutes. After the treatment with bromine, flushing with nitrogen is carried out again for approximately l0 min utes, and the furnace is then allowed to cool. The molybdenum incandescent-lamp element is removed from the molybdenum boat and incorporated in the lamp in known manner.

EXAMPLE 2 EXAMPLE 3 The molybdenum element is fitted in the incandescent lamp, and the semi-finished lamp is filled with a brominecontaining gas. The bromide layer is formed by flashing as it is called, i.e. by passing current through the element for a brief period.

The advantages of the method of the invention are summarized below. The technique is simple, and the bromide can be used in several forms, e.g. elementary bromine and bromine-containing hydrocarbons. The

coated molybdenum element is not affected by any damage to its surface since the molybdenum bromide layer reforms during operation of the lamp. The entire coating process can be carried out using equipment known per se.

What we claim is:

l. A method of producing a surface layer of bromide on a molybdenum element, comprising degreasing the surface of the molybdenum element, heating the degreased surface in a stream of hydrogen at approximately 700C for at east 10 minutes, and then treating such heated surface with bromine-containing gas at about 1000C, and cooling the thus-treated element in a gas.

2. The method according to claim 1, wherein the heated surface is treated with bromine-containing gas at about 1000C for 10 minutes.

3. The method according to claim 1, wherein the cooling of the element which has been treated with bromine-containing gas is carried out in nitrogen.

4. A method according to claim 1, wherein the bromine-containing gas is a hydrocarbon.

S. A method according to claim 4, wherein the hydrocarbon is C H Br 6. A method according to claim 1, wherein the molybdenum element after degreasing and heating in hydrogen as recited is incorporated in an incandescent lamp in semi-finished condition, the semi-finished lamp is filled with bromine-containing gas, and the bromide layer is formed by passing electric current through the 

2. The method according to claim 1, wherein the heated surface is treated with bromine-containing gas at about 1000*C for 10 minutes.
 3. The method according to claim 1, wherein the cooling of the element which has been treated with bromine-containing gas is carried out in nitrogen.
 4. A method according to claim 1, wherein the bromine-containing gas is a hydrocarbon.
 5. A method according to claim 4, wherein the hydrocarbon is C2H2Br2.
 6. A method according to claim 1, wherein the molybdenum element after degreasing and heating in hydrogen as recited is incorporated in an incandescent lamp in semi-finished condition, the semi-finished lamp is filled with bromine-containing gas, and the bromide layer is formed by passing electric current through the element. 